Semiconductor devices typically include a wafer comprising silicon or another semiconductor material that is etched or otherwise processed to form circuit elements. The wafer typically includes surface features, such as electrical contacts or other components, that either project upward from the surface of the wafer or form depressions or concavities in the surface of the wafer. Once the wafer has been processed, it may be desirable to examine the features under a microscope and measure the dimensions of the features to ensure that they conform to design specifications. Because the features are typically too small to resolve with visible light, which has a relatively large wavelength, the wafers are typically examined with short-wavelength electron beams under a scanning electron microscope (SEM).
One conventional method for analyzing a semiconductor wafer with a SEM includes scanning the wafer with an electron beam that is either reflected by the features, or causes the features to emit secondary electrons. The reflected or emitted electrons are received by a detector that generates a voltage signal corresponding to the rate at which the electrons are received. The voltage signal changes as a function of the orientation of the features and may be used to create an image having contrasting light and dark regions, similar in appearance to a conventional photograph. The distances between the light and dark regions may then be measured to determine the dimensions of the features.
Where the features of the semiconductor wafer are spaced relatively far apart, the electron beam may be relatively wide and may have a relatively large depth of focus, so that the electron beam may resolve features having a wide range of heights or depths relative to the surface of the wafer. As electronic devices are made smaller and smaller, the spacing between the features on the surface of the wafer becomes smaller and smaller, and the aspect ratio of the spaces between the features increases. To adequately resolve the closely-spaced features, it has become necessary to reduce the width of the electron beam.
One drawback with conventional electron beam techniques is that, when the width of the electron beam is reduced, the depth of focus of the beam is also reduced. Accordingly, the electron beam may not be accurately focused on any relevant portion of the feature, or may be accurately focused on only one relevant portion of the feature. For example, where the beam is not accurately focused on the edges of the feature, it may be difficult to locate the edges of the feature, and may accordingly be difficult to determine the dimensions of the feature. Where the electron beam is focused on only one portion of the feature, only that portion may be accurately measured. For example, where the feature is tapered and the dimensions of the feature vary with distance from the surface of the wafer, the dimensions may only be accurately measured at the portion of the feature near the focal point of the electron beam.
Electron beams in a SEM may be automatically focused using split beam techniques or other procedures. However, a drawback with conventional SEM automatic focusing methods is that, upon reviewing the image created by the SEM, it may not be clear which portion of the feature is in focus. If the dimensions of the feature change as a function of distance from the surface of the wafer (e.g., if the feature is tapered), it may be difficult to determine which portion of the feature the resulting measurement corresponds to. The resulting measurements may therefore be inaccurate.